Background

Passive restraint systems such as airbags or automotive seat belt tensioners have experienced increased usage in vehicles to protect passengers during frontal collisions. These types of restraints require no extra action by the passenger to achieve protection. Rather, the passive restraint is automatically activated in the event of an activation worthy impact event.

Various types of impact sensors have been designed for determining if passive restraint, such as inflation of the airbag, is needed for a given impact event to ensure that the airbag is inflated only when airbag protection is necessary. Ideally, the impact sensor is able to discriminate between activation worthy events and non-activation worthy events.

One type of impact sensing system uses a plurality of threshold switches in the front region of the vehicle. These switches send an impact signal for inflating the airbag if a high impact event is severe enough to close the switches. Mechanical sensor-based systems of this type too often rely on sensor redundancy to minimize the negative effects of any sensor malfunction which may occur. This requires a large number of switches within each vehicle, increasing the overall complexity of the impact sensing system. Wire harnesses which require additional assembly must also be used in this type of system. Additionally, the threshold switches must be located in strategic places in the vehicle where they have the best opportunity to detect and discriminate between various types of impacts. Determining these locations requires extensive impact testing and studying of impact effects on the vehicle to determine the best placement for the threshold switch sensors.

Another type of impact sensing system uses single-point impact sensors instead of multiple switches. This type of sensor has an accelerometer located in the passenger compartment of the vehicle to constantly monitor the acceleration of the vehicle and sense any sudden deceleration of the vehicle. The output of the accelerometer is continuously analyzed to determine if and when deceleration occurs and if the deceleration is caused by a impact which is severe enough to require activation of the airbag or other passenger restraint. The impact sensing system must be sophisticated enough to prevent inadvertent activation of the safety devices and yet simple enough to test quickly and often to assure proper operation.

A single-point impact sensing system must also be able to quickly and efficiently evaluate the output from the accelerometer to determine whether a given impact requires activation of safety mechanisms. The output of the accelerometer must have characteristics which can be easily analyzed to discriminate between activation worthy and non-activation worthy events. The impact sensor must measure not only the magnitude of the deceleration but also its duration to provide the most accurate response.

Many types of single-point impact sensors use the frequency response of the accelerometer to determine whether a given impact requires activation of passenger restraints. One type of single-point impact sensor is described in U.S. Pat. No. 5,065,322 to Mazur et al. The signal from the accelerometer is sent through an analog-to-digital converter to convert it into a digitized time domain vibratory electric signal. A fast Fourier transform device then transforms the digitized signal into frequency domain signals. The amplitudes of the frequency domain signals are summed over the entire frequency spectrum and evaluated to determine whether there are frequency components which indicate the occurrences of a predetermined type of vehicle impact. The passenger restraint is actuated when the sum of the amplitudes of at least one selected frequency is greater than a predetermined threshold. This type of impact sensing method, as well as the circuitry to perform this method, is quite complex since the accelerometer signal must first be digitized and then converted into the frequency domain before any type of signal evaluation can take place. Transform devices and various filters for eliminating signals outside the desired frequency range must be used to analyze the acceleration signal, increasing the overall number of electrical devices in the impact sensor and decreasing its adaptability for integration onto a single chip.

An accelerometer which provides a digital output is described in U.S. Pat. No. 5,095,750 to Suzuki et al. The accelerometer has a movable electrode in between two fixed electrodes to form two variable capacitors. Acceleration forces displace the movable electrode, changing the size of the gaps in between the movable electrode and the fixed electrodes and consequently varying the two capacitances. A capacitance detector measures the gaps and generates an output voltage which represents the difference between the lengths of the gaps. A pulse width modulator generates an output signal which has a pulse width modulated according to the magnitude output voltage. The pulse width of this output signal is accurately proportional to the acceleration to be detected. This output signal, however, cannot be easily interpreted to determine the occurrence of a vehicular impact condition and therefore the accelerometer is not easily adaptable for use in a single-point impact sensor. Also, the response in this type of accelerometer is susceptible to temperature changes, requiring additional temperature compensation circuitry to offset the changes.

Technology

A single-point impact sensor has a fully differential capacitive sense element for providing a capacitive difference which is proportional to the acceleration of the vehicle. The capacitive difference is converted into a digital pulse train signal which is pulse density modulated with variations in the capacitance. The pulse density of the pulse train is evaluated according to a hierarchy of counters and timers to determine if it is indicative of an activation worthy event. In one embodiment, a non-volatile programmable memory is provided to control the operation of the impact sensor.